This invention pertains generally to communications and guidance systems, and more particularly to a two axis gimbal for controlling a mirror or radar dish in such a system.
Two axis gimbals are used in many applications where a telescope, mirror, or some other instrument has to point or track in two axis space. Such applications include directing lasers from one satellite to another satellite, or accurately positioning a radar dish. Typically, four or six bearing gimbals with complex support systems are used to provide the two degrees of freedom. In addition, gimbals may be required to track in relatively small increments, although gimbals suffer from a number of problems in precision applications.
Two of the more difficult problems with the precision positioning provided by two axis gimbals are reduction of pointing accuracy caused by break away torque, or xe2x80x9cstiction,xe2x80x9d in the motor bearings of the gimbal, and high friction and stiffness associated with the movement of various power and signal cables associated with the gimbal. In precision operations, these restricting forces cause the motor to produce an energy peak, which typically causes the motor to overshoot a desired position. The moving cables also create control and reliability problems due to the stiffness and friction as described above, which change as a function of temperature, time, positioning, and other factors.
Several methods have been developed to reduce the effects caused by the cables. One such method in which the gimbal includes a yoke and a shaft adapted to controllably move relative to the yoke is to cluster the cables along the sides of the yoke. This method, however, provides an undesirably high level of resistance during the operation of the gimbal, with the physical properties of the cables creating varying and unpredictable forces that must be overcome by the gimbal motor. Furthermore, this technique can create hardware interference problems and cause pronounced cable fatigue. For example, cables can become caught in nearby components and may encounter more bending cycles during operation. Still other gimbal designs leave the cables unconstrained, allowing the cables to hang freely from the gimbal. This design, however, can create dynamic loading problems. More specifically, moving cables provide inertia loads that typically change in magnitude with respect to position. Thus, these loads can cause control problems as the gimbal is moved from one position to another.
Another concern with current gimbals is the high number of total parts, which increases production costs, mass, and complexity, while reducing overall product reliability. Two axis gimbals in particular require a large number of parts and a correspondingly large assembly time. As described above, two axis rotation is typically provided by a plurality of bearings, wherein each bearing must be preloaded to a high tolerance to insure proper bearing and shaft alignments. Such gimbals disadvantageously require large amounts of labor time to install, as well as increasing the total part count and overall mass. The latter factor becomes increasingly important when considering use in space applications, where slight weight differences have a dramatic effect on launch costs.
The two axis gimbal of the present invention provides a gimbal having extremely low bearing stiction, nonmoving cables, and a reduced number of total parts. The present invention is applicable for many types of gimbaled instruments, but is particularly adapted for precision applications since a number of limitations inherent in conventional gimbals have been overcome. The two axis gimbal of the present invention includes a spherical bearing, such as a miniball spherical bearing, having extremely low stiction that accommodates the two axis motion. The bearing allows roll, pitch, and yaw motion, although the gimbal is typically designed to exclude yaw motion. The gimbal also includes a shaft that runs through the bearing. The inner race of the bearing therefore allows the shaft to move, typically in roll and pitch, while the outer race of the bearing is fixed to a support structure. The gimbal also includes a drive mechanism, such as a spherical motor, for rotating the shaft within the spherical bearing relative to the fixed support structure. In embodiments in which the gimbal also includes a reflector mounted to the shaft, the drive mechanism serves to rotate the reflector about two orthogonal axes, i.e., pitch and roll.
The gimbal can also include a counterbalance to balance the gimbal. In a preferred embodiment, the counterbalance is eccentric and movably attached near one end of the shaft. The counterbalance may be rotated around the shaft or moved along the shaft to balance the gimbal in the pitch and roll axes. In addition, the gimbal can include pairs of sensors and sensor triggers for monitoring the position of the shaft and, in turn, the reflector relative to the pitch and roll axes.
The design of the gimbal of the present invention permits the cables that connect the drive mechanism to the power source to remain stationary during operation of the gimbal. As a result, the stiffness and friction created by attempting to move the cables of current gimbals is eliminated. Moreover, the spherical motor and bearing combination greatly reduces the overall number of parts, mass and volume, in some cases up to 50%.